A laser is a device which has the ability to produce monochromatic, coherent light through the stimulated emission of photons from atoms, molecules or ions of an active medium which have typically been excited from a ground state to a higher energy level by an input of energy. Such a device contains an optical cavity or resonator which is defined by highly reflecting surfaces which form a closed round trip path for light, and the active medium is contained within the optical cavity.
If a population inversion is created by excitation of the active medium, the spontaneous emission of a photon from an excited atom, molecule or ion undergoing transition to a lower energy state can stimulate the emission of photons of substantially identical energy from other excited atoms, molecules or ions. As a consequence, the initial photon creates a cascade of photons between the reflecting surfaces of the optical cavity which are of substantially identical energy and exactly in phase. A portion of this cascade of photons is then discharged out of the optical cavity, for example, by transmission through one or more of the reflecting surfaces of the cavity. These discharged photons constitute the laser output.
Excitation of the active medium of a laser can be accomplished by a variety of methods. However, the most common methods are optical pumping, use of an electrical discharge, and the passage of an electric current through the p-n junction of a semiconductor laser.
Semiconductor lasers contain a p-n junction which forms a diode, and this junction functions as the active medium of the laser. Such devices are also referred to as laser diodes. The efficiency of such lasers in converting electrical power to output radiation is relatively high and, for example can be in excess of 40 percent.
Conventional laser diodes (as used herein, the term laser diode includes laser diode arrays) are available which produce output radiation having a wavelength over the range from about 630 to about 1600 nm. For example, the wavelength of the output radiation from a GaInP based device can be varied from about 630 to about 700 nm by variation of the device composition. Similarly, the wavelength of the output radiation from a GaAlAs based device can be varied from about 750 to about 900 nm by variation of the device composition, and InGaAsP based devices can be used to provide radiation in the wavelength range from about 1000 to about 1600 nm.
For the purposes of this application, the terminology "GaInP based laser diode" refers to any laser diode or laser diode array wherein the active medium of the device is comprised of at least one material selected from the group consisting of: (a) alloys of gallium, indium and phosphorus, and (b) alloys of gallium, indium, aluminum and phosphorus.
Laser diodes having an active layer consisting of either a GaInP alloy or a GaInAlP alloy, for example Ga.sub.0.5 In.sub.0.5 P, have the ability to produce visible light at a wavelength in the range from about 630 to about 700 nm, and more typically in the range from about 670 to about 700 nm when operated at a temperature of about 25.degree. C. AlGaInP alloys, which can be lattice-matched to GaAs, have been used as cladding layers for GaInP alloy active layers. The preparation and operation of double-heterostructure GaInP based laser diodes on GaAs substrates have been reported as follows: (a) Ga.sub.0.5 In.sub.0.5 P active layer and Al.sub.0.2 Ga.sub.0.3 In.sub.0.5 P cladding layers, Kobayashi et al., Electron. Lett., Vol. 21, No. 20, pp. 931-932 (26 Sept. 1985); (b) Ga.sub.0.52 In.sub.0.48 P active layer and Al.sub.0.26 Ga.sub.0.26 In.sub.0.48 P cladding layers, Ikeda et al., Appl. Phys. Lett., Vol. 47, No. 10, pp. 1027-1028 (15 Nov. 1985); (c) Ga.sub.0.5 In.sub.0.5 P active layer and Al.sub.0.25 Ga.sub.0.25 In.sub.0.5 P cladding layers, Ishikawa et al., Appl. Phys. Lett., Vol. 48, No. 3, pp. 207-208 (20 Jan. 1986) and Ikeda et al., J. Cryst. Growth, Vol. 77, pp. 380-385 (1986) and (d) In.sub.0.5 (Al.sub.0.2 Ga.sub.0.8).sub.0.5 P active layer and In.sub.0.5 (Al.sub.0.9 Ga.sub.0.1).sub.0.5 P cladding layers, Dallesasse et al., Appl. Phys. Lett., Vol. 53, No. 19, pp. 1826-1828 (7 Nov. 1988).
Laser diodes respond to changes in temperature in a variety of ways. For example, the peak wavelength of near infrared GaAlAs devices can be varied by as much as about 5 or 10 nm simply by adjusting the temperature of the device over the temperature range from about 20.degree. C. to about 50.degree. C. In addition, the lifetime of a laser diode is a function of temperature. Indeed, the lifetime of such a device can decrease by an order of magnitude in response to a 40.degree. C. rise in temperature. Finally, the power output of a laser diode at constant drive current is a function of temperature and will usually increase as the temperature is lowered. However, for a typical near infrared GaAlAs device operated at a constant drive current, this increase in power will generally be less than 1% per degree Centigrade. Thus, such a device, when operated at 0.degree. C., would have an increase in output power of less than 25% when compared to the output power of the same device at 25.degree. C. when operated at a constant drive current.
The above-cited Kobayashi et al. reference sets forth data for the described Ga.sub.0.5 In.sub.0.5 P device illustrating optical output power versus DC and pulsed driving current at various temperatures over the range from 15.degree. to 80.degree. C. Similarly, the above-cited Ishikawa et al. reference sets forth data for the described Ga.sub.0.5 In.sub.0.5 P device illustrating light output power versus DC driving current at various temperatures over the range from 24.degree. to 51.degree. C. Further, the above-cited Dallesasse et al. reference sets forth data for the described In.sub.0.5 (Al.sub.0.2 Ga.sub.0.8).sub.0.5 P device illustrating continuous (cw) light output versus current at various temperatures over the range from 20.degree. to -30.degree. C. In each case, a reduction in temperature resulted in increased output power at a given driving current. However, none of these references either teaches or suggests operating such a device in an environment having an ambient temperature in the range from about 10.degree. to about 40.degree. C. and simultaneously cooling the device thermoelectrically during such operation by an amount which is sufficient to at least double its power output relative to that obtained at ambient temperature and constant drive current.
U.S. Pat. No. 3,840,889 (O'Brien et al., Oct. 8, 1974) is directed to a laser diode device which has a low inherent inductance when operated at high frequencies. It is disclosed that it may be necessary to lower the temperature of the laser diode to an operating temperature below that of room temperature in order to operate the diode at a low threshold current.
U.S. Pat. No. 4,238,759 (Hunsperger, Dec. 9, 1980) is directed to a Peltier-cooled laser diode wherein the cooling junction is located within a few microns of the active layer of the diode. This reference teaches that, in semiconductor lasers, temperature is a limiting factor for line width, wavelength stability, operating lifetime and threshold current. It is further disclosed that the wavelength of the laser output radiation can be tuned by temperature variation.
U.S. Pat. No. 4,315,225 (Allen, Jr. et al., Feb. 9, 1982) discloses that semiconductor laser diodes have been operated to yield a continuous output of as much as 40 mW at room temperature with much higher powers being obtained at lower temperatures.
U.S. Pat. Nos. 4,338,577 (Sato et al., July 6, 1982) and 4,604,753 (Sawai, Aug. 5, 1986) both teach the combination of a laser diode with a thermoelectric heat pump and disclose that the optical output power from the diode decreases with an increase in temperature.